Even a simple movement with both arms can change when gravity decreases or disappears. During flights that briefly reproduced weightlessness, volunteers had more difficulty maintaining certain coordinated movements.
An astronaut often has to use both hands at the same time to operate controls, tools, or equipment. On the Moon or Mars, however, their body will be subjected to only a fraction of Earth's gravity.
ESA astronaut in the ISS, Paolo Nespoli
ESA illustration
To measure this effect, the team used parabolic flights. The aircraft follows a trajectory that creates different levels of gravity for several dozen seconds at a time. Participants therefore performed the same exercises under Earth's gravity, in weightlessness, and at several intermediate levels.
The test required producing rhythmic movements with both arms according to different relationships. Other exercises imposed a delay between the movements, requiring greater control to maintain the requested rhythm.
It was during these less natural movements that the absence of gravity had the clearest effect. In weightlessness, the gestures became more chaotic and gradually drifted toward the movement in which both arms act together. In other words, when their usual reference points disappeared, the motor system more easily returned to a simple organization.
Reduced gravity did not, however, produce the same result as its complete absence. The experiments also reproduced 25%, 50%, and 75% of Earth's gravity. As gravity increased, participants generally regained better precision and less variable movements during the most difficult exercises.
This result shows that gravity does more than keep the body on the ground. The brain and muscles constantly use gravity, their natural environment, to organize movements. Changing this environment can therefore alter how several gestures are coordinated, even when the person knows perfectly well what they are supposed to do.
The researchers also published a complementary study on the force produced simultaneously by both arms. It indicated that weightlessness notably reduced the regularity of this force. Once again, intermediate gravity levels generally produced performances closer to those observed on Earth.
These experiments remain very brief compared with a stay in space. They therefore cannot reveal how the brain would adapt after several days or months. The authors particularly want to study longer exposures and different levels of partial gravity, in order to better prepare for tasks performed on the Moon or Mars.